Only if the power specifically says you 'must' push them. Otherwise, forced movement is completely optional; X squares, fewer than X squares, or not at all.

And if the power specifies both a distance to push and a destination, you can push that distance (or less) to put the target in the specified destination... or you can not push at all. But you can't push to a position that doesn't qualify for the destination.

Also, you don't have to push in a straight line; you just have to make sure that every square of forced movement qualifies as a push.

All of this applies to pulls and slides as well.

(This is why they've been going through and fixing the cases where a power grants a slide of unspecified distance to a destination. Per RAW, with such a power you can slide the target completely around the battle area three or four times and then to the destination. This has been abused on occasion. So they've been adding distance restrictions.)

"The world does not work the way you have been taught it does. We are not real as such; we exist within The Story. Unfortunately for you, you have inherited a condition from your mother known as Primary Protagonist Syndrome, which means The Story is interested in you. It will find you, and if you are not ready for the narrative strands it will throw at you..." - from Footloose